RcParams instances for matplotlib.style.use

External libraries may want to prepare styles dynamically. For example, `brewer2mpl` provides various colormaps. While it could create it's own context manager for temporarily changing the `color_cycle`, I think it makes more sense if `matplotlib.style.use` can work directly with `RcParams` instances.

```
>>> with matplotlib.style.context(matplotlib.RcParams(**{'text.usetex':False})):
...     print matplotlib.rcParams['text.usetex']
...      
False
```
Want this instead?

```
        if cbook.is_string_like(style):
            if style in library:
                mpl.rcParams.update(library[style])
            else:
                try:
                    rc = rc_params_from_file(style, use_default_template=False)
                    mpl.rcParams.update(rc)
                except:
                    msg = ("'%s' not found in the style library and input is "
                           "not a valid URL or path. See `style.available` for "
                           "list of available styles.")
                    raise ValueError(msg % style)
        else:
            mpl.rcParams.update(style)
```